1. Field of the Invention
The invention relates to a semiconductor device manufacturing process. More particularly, the invention is a process for forming an Ohmic contact on a wide bandgap semiconductor material (SiC). More particularly, the invention is a process for forming a nickel silicide (Ni2Si) Ohmic contact to n-type SiC.
2. Discussion of the Related Art
The invention relates to the technology of manufacturing a SiC based semiconductor device. All semiconductor devices require high quality, low resistivity thermally stable reliable n- and p-type Ohmic contacts. Ohmic contacts are essential for the transfer of signals between the SiC device and the external circuitry. Ohmic contact metallization design, selection and method of deposition heavily influence the electrical and physical properties of Ohmic contacts to all semiconductors. There are many metallization designs; however, the most common used in SiC device technology consists of single layer, bilayer, multilayer composite metallization schemes and compound intermetallics. Low resistance Ohmic contacts to n-type SiC can be obtained by depositing metallizations with appropriate work functions to achieve a barrier height reduction at the metal-SiC interface. The barrier heights of various contacts depend on the surface properties and electron affinity of the semiconductor, and the work function of the metal. For SiC, barrier heights do not follow the Schottky Mott theory and are typically constrained to high values. In practice, the degree of dependence of barrier height on choice of metal has been determined to be dependent on the bulk semiconductor properties, the nature of the surface before contact formation and the interfacial chemistry. At practical SiC interfaces, interface states cause Fermi level pinning which results in the formation of a potential barrier. The interface states affect the charge transfer between the metal and the SiC, hence control the Schottky barrier height.
Metal deposition can be achieved by a variety of methods; the most common being electron-beam (e-beam) evaporation, sputtering, and thermal evaporation. Sputtering and e-beam evaporation is preferred over thermal evaporation for Ohmic contact metal deposition. The reason for this centers on the fact that thermal evaporation methods involve either a resistance-heated source (refractory metal coiled into a filament) or an evaporation source that is heated by rf induction (utilizing a BN crucible) which results in contamination of the charge from the heater or crucible. This contamination leads to unwanted impurities which adversely affect the electrical properties of the condensed Ohmic contact metal film. The method of metal deposition heavily influences the grain size, uniformity, and metal-epilayer interface properties of the condensed contact film. Grain boundaries serves as diffusion pathways, thus minimization of grain boundary area is desirable to control unwanted elemental diffusion. For the same metal deposited by sputtering and e-beam evaporation, the e-beam evaporated films will possess a much larger grain size, which translates to less grain boundary area and lower diffusion pathways. Additionally, larger grain metal films possess higher hardness values which influence the overall mechanical durability of the metal film. E-beam evaporated metal films also have less gas incorporation in the condensed films than sputtered metal films, and as a result have a higher level of film purity. However, the film-substrate adhesion is usually enhanced for sputtered films since the condensing atoms have higher energy with respect to e-beam deposited films. Both e-beam and sputter deposition produce large area uniform films. The method of metal deposition is often determined by the choice of metal to be deposited, for example, since refractory metals are not easily deposited via e-beam evaporation, sputter deposition is preferred. Additionally, the metal deposition method is usually a strong function of equipment availability.
Deposition of an intermetallic Ohmic contact such as X2Y can be accomplished by sequential e-beam evaporation of each component layer (X and Y) separately with the correct thickness so as to achieve the correct stoichiometric ratio of 2X:1Y followed by a post metal deposition annealing step to achieve the uniform intermetallic phase, X2Y. Alternatively, the intermetallic phase X2Y can be produced by the sequential co-sputtering of two source targets (target #1 is composition X and target #2 is composition Y) such that two layers, X and Y, are deposited with the proper thickness and post metal deposition annealed in order to achieve the uniform intermetallic phase X2Y with the correct stoichiometric composition 2X:1Y. A single compound target with composition X2Y can also be sputter deposited onto the SiC substrate or SiC substrate with SiC epilayer, however the proper 2X:1Y stoichiometry is not easily achieved via this deposition technique. The physical vapor deposition technique, pulsed laser ablation deposition (PLD) has been extensively employed for the preparation/deposition of high quality thin films of multi-component metal-oxide ceramics. The principle desirable feature of PLD is that it is a non-equilibrium evaporation process which produces an intense plasma plume and transfers the target composition/stoichiometry (particularly metallic constituents in the multi-component systems) into the deposited film on a substrate. Thus, PLD is an excellent method for depositing an intermetallic phase X2Y (Ni2Si) onto a substrate (n-SiC) or SiC substrate with SiC epilayer where precise stoichiometric composition is necessary to achieve a highly conductive low resistivity metal film for Ohmic contact formation (such as Ni2Si-n-SiC).
Post metal deposition annealing is usually required to achieve Ohmic behavior in the deposited metal films on SiC. Annealing of metal contact-SiC structures has been achieved via conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The choice of ambient annealing gas influences the electrical properties of Ohmic contact significantly. For CFA, the ambient atmosphere can range from vacuum annealing where metal oxidation is an issue, to flowing N2, Ar, and forming gas. The most common RTA ambient gases are N2, Ar, and forming gas. CFA requires longer processing times with respect to RTA and thus often enhances unwanted elemental diffusion. For annealing some contact metals to SiC the thermally induced interfacial reactions which are responsible for Ohmic behavior demand narrow time-temperature process windows, thus favoring rapid thermal processing. Specifically, RTA is a fast thermal processing method, 1-100 sec, which provides the advantages of (a) reduced thermal budget (anneal temperature×anneal time), and (b) rapid heating and cooling rates in the range of 30-500° C./sec. These advantages are critical to the formation of good Ohmic contacts since the desired interfacial phase formation usually occurs at some temperature (T1) for a time (t1), after which the processing temperature can be reduced rapidly by as much as 50° C. in 1 sec. Thus, undesired contact metal-SiC phase formation/phenomena having different activation energy than the desired contact metal-SiC phase formation are almost completely suppressed since only a fraction of a second would be available for the undesirable process or phase formation. The most common annealing method for SiC contact technology is RTA, but again, like metal deposition methods, the annealing mode is often a function of equipment availability.
Optimum contact performance is not only influenced by the material processing and design elements described above, but also relies on strict adherence to a set of fundamental contact-semiconductor requirements. Currently, there are several critical requirements for Ohmic contacts which must be satisfied in order to achieve high performance reliable SiC based devices. The most prominent of these is the attainment of a reproducible low specific contact resistance value. Additionally, the Ohmic contact must not significantly perturb device performance. In other words, the contact must supply the required current density with a voltage drop that is significantly small compared with the drop across the active region of the device. The contact must also possess good mechanical properties, that is, good metal adhesion during formation, subsequent processing and in service device operation. The metallization must not cause excessive stress in the underlying semiconductor since this can result in alteration of electronic characteristics. The Ohmic contact must be temporally and thermally stable. Additionally, the Ohmic contact must be environmentally stable under prolonged bias-temperature stress, humidity, and reactive ambient conditions. The contact metal-semiconductor interface must be uniform, shallow, and abrupt. In the case of alloyed contacts, the metal-semiconductor interface phase(s) must be laterally homogenous. This uniformity of metal-semiconductor interfacial reactions serves to minimize the spread in contact resistance values, suppress current nonuniformity and improve device reliability. The metallization must have a smooth surface morphology, which is critical for device wire bonding. Fabrication of the Ohmic contacts must be controllable and reproducible, that is, reactions and other properties which govern the contact resistance must be reproducible and the contact fabrication must be compatible with semiconductor processing. Finally, the drive towards lower contact processing temperatures should be adhered to whenever possible. This is critical for device integration issues, namely, integration of materials with differing thermal stability within a single device (heterostructures) and/or integration of several devices, composed of different materials, on a common substrate.